The use of hybrid hidraulic models in the process of hydropower plants design on the lower Sava

In complex spatial planning cases of positioning water infrastructure such as hydropower plants, which impact longer river sections and could significantly change the run-off regime and flood hazard in the influential area, accuracy and reliability of modelling are crucial. Therefore, hydraulic analyses with both physical and mathematical models are used. Hybrid hydraulic models combine the benefits of a physical and a mathematical model: a physical model can still more precisely provide data where the influence of turbulence and other 3D flow phenomena are significant, while a mathematical model upgrades the physical model in the phase where significant number of variant analyses, planning changes and data presentations for decision support are required. Such an approach was also used in planning the exploitation of hydropower potentials of the Sava River in the section between Krsko and the state border with Croatia, where the complexity of morphological, hydrological, hydraulic and anthropogenic factors is significant. Beside regulating the run-off regime, related to operation of the hydropower plant, it was also necessary to consider the conservation of the existing run-off flow regime around of the water intake structure for the Nuclear power plant Krsko and conservation of the existing run-off regime from the study area downstream of the Sava-Krka confluence. In the first phase the model was used for analysing the existing state (the run-off regime, current retention capacity and flood hazard in the influential area of the planned hydropower plants), whereas in the second phase the design variants of HPP Brežice and Mokrice and all other planned encroachments and measures for assurance of flood safety in the influential area were analysed. In the design process, we optimized the location of left and right bank dike, the dimensions and location of the flood water overflow structures and the dimensions of the HPP weir structure. Thereby, the current flood and erosion hazard and risk are for the most part reduced while the current run-off regime and water retention capacity are preserved.